Your bike might have braze-on fittings for holding the current cables in place. These won't fit the hydraulic line because the opening will be too small, and the braze-ons won't be wide enough to accept the line through the wire-slot. And you can't generally disconnect the hydraulic line to feed through the braze-on anyway.
There's nothing wrong with using cable ties to fasten down the hose/pipe instead, and you can run the new line beside your braze-ons for some support.
On the other hand you might wish to grind off the braze-ons for a more streamlined look. Bad idea on carbon and even aluminium I'd not do it. But a steel frame is easy to work on. Just touch up the bare metal after to avoid rust.
Internally routed cables suffer the same problem as braze-ons. The entry/exit holes are generally too small, and you have to disconnect the hose anyway to get in them,
But you might want something that looks better.
Option 1. http://problemsolversbike.com/products/brakes/clamp_on_cable_guides_-_27723
These are intended for top tubes but may fit your fork.
Option 2. Stick-on cable guides
There's a bunch of different brands and styles, these are Alloy ones.
And these are plastic ones. You'd have to clean the frame really well to ensure a good adhesion, but there's fairly low forces acting on the tube while braking - all the pressure is internalised.